This invention relates to a semiconductor device and specifically to a semiconductor device having a package structure which has been improved so that it can be applied to an ultra-high frequency circuit or high speed digital circuit.
Along with the rapid progress in improvements in data processing system operation speed and capacity, efforts have been made continuously toward development of a semiconductor device to be applied to data processing which realizes operation at a higher frequency and higher electrical power.
For example, as an active semiconductor device, a semiconductor element consisting of a compound semiconductor such as gallium-arsenide (GaAs), for example, a gallium-arsenide Schottky barrier type field effect transistor, have been applied in place of existing semiconductor elements consisting of silicon (Si). Such gallium-arsenide Schottky barrier type field effect transistors have been further improved so that they can now operate at a higher frequency and provide a higher electrical power.
On the other hand, packages for accommodating such a semiconductor device operating at a higher frequency and electrical power, having the structure shown in FIG. 1 through FIG. 12, have been proposed.
FIG. 1 is a perspective view of the main part of a ceramic frame type package, wherein 1 is a copper (Cu) base; 2 is a ceramic case; 2A is a first ceramic frame; 2B is a second ceramic frame; 3 is a lead segment; and 4 is a metallized film for sealing, respectively.
FIG. 2 is a perspective view of the main part of a ceramic plate type package, wherein 5 is a copper base; 6 is a ceramic plate; 6A is a window for accommodating a semiconductor element; and 7 is a conductive film, respectively.
These packages are now widely used because they are small in size and cheap to use as a package for a semiconductor element or small-size solid state apparatus. An attempt to obtain a higher performance package has encountered various problems.
For example, in the case of the structure shown in FIG. 1, when an element is hermetically sealed by securing a ceramic cover using a metallized film 4, the ring-shaped metallized film 4 on the ceramic frame 2B may resonate. Such resonance occurs at a frequency determined in accordance with a circumference of the ring-shaped metallized film 4. When a metal cover or a ceramic cover which is metallized is used for sealing, the resonance frequency can be raised a little but resonance occurs in accordance with the area of metallized part. Therefore, this type of package is restricted to relatively low frequency use and is generally not used in the ultra-high frequency situations for this reason.
Moreover, the package with a structure as shown in FIG. 2 is also not suitable for ultra-high frequency or speed because crosstalk may occur between the conductive films 7, however this package is designed for a high speed digital circuit. Recently, the requirement for high speed digital circuits also has urged development of circuits particularly for high speed operation, and accordingly circuits operating in the GHz band are now appearing in the Si based systems as well as in the GaAs based system elements. These circuits, for example, logic circuits and memory circuits, have complicated functions and have input/output terminals requiring as many as 10 to 40 pins. Improvement in high speed operation of these logic and memory circuits is accompanied by other problems in addition to the sealing problem and crosstalk mentioned above, for example, signal leakage due to a combination of signals between terminals due to existence of many terminals within a restricted area, resonance and oscillation. In addition, high speed operation is degraded by an increase of the parasitic capacitance at the sealing area.
Circuits for use in an ultra-high frequency band are currently formed on a carrier manufactured by laminating copper and a ceramic, the carriers are placed within a metal case type package as shown in FIG. 3 or FIG. 4, and the entire assembly is sealed to protect the circuits from ambient environmental conditions. FIG. 3 is a perspective view of a metal case type package, whereby 8 is a metal case; 9 is a coaxial connector; 9A is a core conductor; and 10 is an insulator consisting of ceramic or resin, respectively. FIG. 4 is a perspective view of a metal case type package with brims, wherein 11 is a metal case with brims; 12 is an insulator; and 13 is a core conductor, respectively. These packages provide disadvantages in that they occupy more space in actual practice than their size indicates and such disadvantages as well as others are explained in detail below.
In FIG. 3, the coaxial line is provided at the sealing part, connection with external circuit is carried out through the coaxial connector 9 and connection with internal circuits is carried out through the extended core conductor 9A. Therefore, as shown in FIG. 5, a chip including a strip line 9B of 50.OMEGA. is formed on the ceramic plate 10A, the core conductor 9A and strip line 9B are connected and the strip line 9B and pad of the internal circuits (the chip) are lead-bonded resulting in a connection of the internal circuits which have the strip line structure. This structure provides a merit in that the coaxial connector 9 can be easily coupled to the coaxial connector of an external circuit but also provides many demerits when trying to realize a small size, high reliability and low cost which are all currently required for a semiconductor device and particularly provides the following two very serious problems.
(i) It is difficult to realize a substantial size reduction because a coaxial connector is used. In attempting to solve such a problem, the core conductor 13 is connected to the strip line of an external circuit. This structure certainly provides a merit in that the coaxial connector can also be used as required but it is difficult to guarantee the characteristic of the external coaxial/strip line conversion since the external conversion to the strip line is performed by users, and it is also difficult to realize impedance matched conversion circuits for a wider operating frequency band.
(ii) Hermetic sealing is difficult because the time required for sealing is longer. Generally, in brazing sealing, a brazing material is placed on the area to be sealed, the cover is then placed thereon, the pieces are held together with a required force and then they are put into a thermostatic oven. The brazing material melts when it reaches the melting point and penetrates into the sealing area and metal of the cover, thus resulting in hermetic sealing. However, in this case, a large amount of gas within the package, for example, nitrogen, is heated and expands, applying pressure to the brazing material from the inside. This causes pin-holes in the sealing area. Moreover, when the package is hermetically sealed by welding, it is difficult to keep constant welding conditions and the welding must be done individually, requiring a long time. Accordingly, yield deteriorates, and manufacturing costs rise. Welding will become particularly more difficult as packages become smaller.
On the other hand, the inventors of this invention have attempted a modification, for example, as shown in FIG. 6, to the package of FIG. 1. In FIG. 6, the same portions as those in FIG. 1 are given the same reference symbols. The metallized films 14, 15, 16 and 17 are formed (coated) on the inside and outside of the ceramic case 2 and both sides of lead segments 3, and moreover these are in contact with the metallized film 4 on top of the ceramic case 2. The effective length of the ring consisting of metallized film 4 is curtailed because of its indirect contact with base 1. However, such a package cannot be used in a high frequency range even with such a modification because of the influence produced by parasitic capacitance and parasitic inductance of the added metallized films.
In addition, the same inventors have also manufactured an ultra-miniature package as shown in FIG. 7. In FIG. 7, the portions that are the same as those in FIG. 1 are given the same reference symbols. This prior art FIG. 7 is different from that of FIG. 1 in that it provides high frequency input terminal, high frequency output terminal 19 and DC input terminals 20, 21, 22 and 23. Moreover, a balanced type amplifier formed as an ultra-miniature circuit on a sapphire substrate is contained within the package, although it is not shown. This device has a merit in that it is small in size and is hermetically sealed, and since the RF input and output terminals of the circuit on the sapphire substrate are matched to the characteristic impedance by using hybrid couplers formed on the sapphire substrate, it is also possible to directly connect a plurality of amplifier stages hermetically sealed by housing the circuits in a package as shown in FIG. 8 and, lead-bonding the terminals on the ceramic frame 2A within the package to the external connecting terminals of the circuits on the sapphire substrate.
In FIG. 8, the three semiconductor devices sealed in the package shown in FIG. 7 are directly connected and the portions that are the same as those in FIG. 7 are given the same reference symbols. In FIG. 8, when high frequency input power is applied to the terminal 18 and DC voltages to the terminals 20, 22, respectively, these are then further supplied to the second and third stages and thereby a high frequency output can be obtained from the terminal 19. This device provides good performance in the 4-8 GHz the range. When using this amplifier in 3-18 GHz range the ring shaped metallized film provided for hermetical sealing resonates in spite of the treatment performed as in the case of the package shown in FIG. 6. The resonant frequency is about 11 GHz which is within the range of 3-18 GHz and an 8-18 GHz amplifier can not be realized. The earth or ground electrode of this package presents another problem as will be discussed later.
Currently, an internal matching type GaAs field effect semiconductor device as shown in FIG. 9 has been manufactured. In FIG. 9, the portions which are the same as those in FIG. 1 are given the same reference symbols. This device provides within the package, the semiconductor chip 24, input matching circuit 25 and output matching circuit 26. The impedance matching is performed within the package for users for whom it is difficult to form the circuit using a semiconductor chip 24 which is a GaAs-FET, and thereby users are only required to prepare and provide a simple external matching circuit, to obtain the desired characteristic, and a DC bias circuit. Accordingly, the device as shown in FIG. 9 is widely used because the desired characteristics can be easily obtained.
However, this package has a disadvantage in that satisfactory performance cannot be obtained when frequency becomes high because the impedance of the GaAs-FET becomes low and moreover parasitic capacitance, parasitic inductance and high frequency loss in the package increase. In addition, because of the resonance of the ring-shaped metallized film as explained above, and the problem related to the earth electrode to be explained later, currently usable packages are limited to the frequency of about 10 GHz. Therefore, a package which can be used for a frequency band exceeding 10 GHz wherein the above-mentioned situation becomes more serious has long been desired. Some packages, however, are currently used in the frequency range of 10 GHz or higher. Such a very small-size package is used for a frequency up to 12 GHz but it is restricted in its application because of its small size and is used only for a low noise GaAs-FET.
Now, the problem of the earth or ground electrode which is in common to any ceramic frame type package will be explained. In general, a package used in a high frequency band has been associated with a problem of cracks generated in the ceramic part due to a difference in thermal expansion coefficients between the ceramic and metal portions. A package shown in FIG. 10 has been proposed in order to prevent such cracks. In FIG. 10, the portions which are the same as those in FIG. 1 are given the same reference symbols. As understood from the figure, the ring 5A is formed on the base 1 and the ceramic frame 2A is mounted thereon. 27 is the metallized film.
This structure provides excellent crack prevention. However, if the frequency becomes higher than 10 GHz, the inductance of ring 5A which exists between the metallized film 27, which is the earth electrode of the ceramic frame 2A and the base 1, becomes larger. Therefore, the metallized film 27 which works as the earth electrode loses its function as the earth electrode and the impedance of the strip line deviates from the optimum value, deteriorating performance.
When a package as shown in FIG. 10 is coupled as indicated in FIG. 11, there is a disadvantage in that a cavity is formed in the area indicated by the arrow and a resonance at the frequency of about 8 GHz occurs. These devices can be cascaded by connecting the packages to each other and bonding at 28 the lead segments 3 as shown in FIG. 11.
As a structure of an electric terminal of a ceramic frame type package, the structure shown in FIG. 12 was proposed by the inventors. In FIG. 12, the portions which are the same as those in FIG. 1 are given the same reference symbols. In FIG. 12, annular metal materials 29, for example which are made of metallizing materials, are disposed on the ceramic case 2 arranged on the base 1 in such a way as surrounding the ceramic portion through which the lead segments 3 are extended from the walls of said ceramic case 2, and said annular metal materials 29 are connected electrically to the base 1. Such a device exhibits better characteristics for the frequency band of 4-8 GHz, since the electric terminal portion provides a pseudo coaxial cable with the annular metal material 29 used as the earth conductor. However, for example, in the 8-18 GHz band, the desired performance cannot be obtained because of parasitic capacitance between the annular metal material 29 and metallized film 4 for sealing. In addition, the annular metal material 29 is not perfectly grounded at a high frequency, for example, as high as 10 GHz or more and produces an inductance. In the extreme case, the annular metal material functions as an antenna.
As will be obvious from the above explanation, the production of packages which can be used for a frequency of 10 GHz or higher is desired for ultra-high frequency circuits and high speed digital circuits. This situation is becoming more serious day by day because recent packages employ the structure of strip line (connection with external circuit)--strip line (sealing part) strip line (connection with internal circuit).